I know that feeling. I'm only 25, working a job I love, but only about 4-9 hours a week, and I have a pretty bad knee. After about four years of it hurting nearly every single day, it just randomly stopped. I don't understand it at all. Everyone once in awhile I feel that it's coming back though.
When I shave my beard, I get carded for R rated movies. It's got to the point though were I constantly think about how fast life is going by and how I'm going exactly no where.
Well, at least you're working. After I graduated, my university degree didn't help me to get a job. I've been unemployed for periods up to 13 months. Good thing I'm working now.
But I wouldn't feel too bad if I were you, I got my first comic published when I was 27, and I'm still contributing to indy anthologies, hoping that one day I'll be a part of a more ambitious project. So, you still have at least a couple of years to try new stuff.
And with your good knee there's no stopping you!